DE69612828T2 - Super alloy based on nickel with optimized platinum-aluminide coating - Google Patents

Description

This invention relates to nickel-based superalloys used in high temperature applications and, more particularly, to articles made from such materials having an optimized platinum aluminide protective coating.

In an aircraft gas turbine, air is drawn into the front of the engine, compressed by a compressor mounted on a shaft, and mixed with fuel. The mixture is burned and the resulting hot exhaust gases are passed through a turbine mounted on the same shaft. The gas flow turns the turbine, which in turn turns the shaft and supplies power to the compressor. The hot exhaust gases flow out the rear of the engine, propelling it and the aircraft forward.

The hotter the exhaust gases, the more efficient the jet engine will operate. There is therefore an incentive to increase the temperature of the exhaust gases. However, the maximum temperature of the exhaust gases is usually limited by the materials used to make the turbine blades and vanes. In current engines, the turbine blades and vanes are made of nickel-based superalloys and can operate at temperatures up to 1,0381,149ºC (1,900-2,100ºF).

Many approaches have been used to increase the operating temperature of turbine blades and vanes. The compositions and processing of the materials themselves have been improved. Physical cooling techniques have been used. In one widely used approach, internal cooling channels are created within the components and cooling air is forced through the channels during engine operation.

In another approach, a metallic protective coating or a ceramic/metallic thermal barrier coating system is applied to the turbine blade or vane component, which acts as a substrate. The metallic protective coating is useful in applications in an intermediate temperature range. One common type of metallic protective coating is a platinum-aluminide coating, which is formed by depositing platinum and aluminum on the surface of the substrate and then diffusing these components into the surface of the substrate.

The thermal barrier coating system is useful in high temperature applications. The thermal barrier coating system includes a ceramic thermal barrier coating that insulates the component from the hot exhaust gas, allowing a hotter exhaust gas than would otherwise be possible with the particular material and manufacturing process of the component.

Ceramic thermal barrier coatings usually do not adhere well directly to the superalloys used in the substrates. Therefore, an additional metal layer, called a bond coat, is placed between the substrate and the thermal barrier coating. The bond coat is usually made of a nickel-containing overlay alloy, such as NiCrACY or NiCoCrAlY, a composition that is more resistant to environmental degradation than the substrate. The bond coat can also be made of a diffusion nickel aluminide or platinum aluminide, the surface of which oxidizes to a protective alumina skin.

While superalloys coated with such metallic protective coatings or ceramic/metallic thermal barrier coating systems provide significantly improved performance over uncoated materials, they still have room for improvement in elevated temperature performance and environmental resistance. There is therefore still a need for improved metallic protective coatings and bonding coats to protect nickel-based superalloys in elevated temperature applications. This need has become more apparent with the development of the latest generation of nickel-based superalloys, as the older protective coatings often do not perform satisfactorily on these materials. The present invention meets this need and provides other related advantages.

EP-A-733 723 (which is prior art under Article 54(3) EPC) discloses a nickel- or cobalt-based superalloy substrate with an average concentration of 8 to 35 wt% platinum and 18 to 28 wt% aluminium deposited thereon.

WO 95/23243 discloses a platinum-aluminium coating in which the platinum is pre-diffused into a nickel-based substrate before the diffusion of aluminium and silicon takes place. No compositions for the coating are given.

The present invention provides a metallic topcoat for nickel-based superalloys. The topcoat is a platinum aluminide useful as a metallic protective coating or as a bond coat for the thermal barrier coating system. The topcoat is in the form of a surface region that is well bonded to the substrate. The platinum aluminide coating has good stability at elevated temperature and resistance to environmental degradation in typical gas turbine applications.

According to a first aspect of the invention there is provided an article having a platinum aluminide surface region comprising a substrate having a nickel-based substrate mass composition and a substrate surface and a platinum aluminide surface region on the substrate surface formed by depositing a layer of platinum on the substrate surface and diffusing the platinum layer into the substrate surface and thereafter providing an aluminum source and diffusing aluminum into the substrate surface, the surface region having an integrated aluminum content of 18 to 24 wt.% and an integrated platinum content of 18 to 45 wt.%, the remainder being components of the substrate mass composition totaling 100 wt.%, the platinum and aluminum content being relatively high adjacent to the substrate surface and decreasing with increasing distance into the substrate from the substrate surface. Preferably the surface region has an integrated aluminum content of 21 to 23 wt.% and an integrated platinum content of 30 to 45 wt.%. All compositions reported herein for the surface regions were determined by an integration technique, discussed in more detail below, which effectively determines an average composition of the surface region. Optionally, a ceramic layer is overlaid on the surface region to produce a thermal barrier coating system.

According to a second aspect of the invention there is provided a method of making an article having a platinum aluminide surface region comprising the steps of providing a substrate having a bulk substrate composition of a nickel-based alloy and having a substrate surface, then depositing a platinum layer on the substrate surface, then diffusing platinum from the platinum layer into the substrate surface, then providing an aluminum source and then diffusing aluminum from the aluminum source into the substrate surface for a time sufficient to create a substrate region at the substrate surface, the substrate region having an integrated aluminum content of 18 to 24 wt.% and an integrated platinum content of 18 to 45 wt.%, the remainder being components of the bulk substrate composition. Optionally, the substrate and the surface region may be annealed and/or a ceramic layer may be deposited overlying the surface region.

Platinum aluminide protected surface areas were previously known, but the present approach creates an optimized platinum aluminide coating that has improved elevated temperature performance and environmental resistance compared to previous platinum aluminide coatings. In addition, the platinum aluminide coating of the invention can be used on advanced nickel-based superalloys without excessive coating growth during use, surface roughening, generation of undesirable phases during use, or reduced stress rupture properties.

The invention will now be described in more detail by way of example with reference to the drawing, in which:

Fig. 1 is a perspective view of a gas turbine component,

Fig. 2A is a schematic sectional view through the component of Fig. 1 along the line 2A-2n, showing an embodiment of the invention,

Fig. 2B is a schematic sectional view through the component of Fig. 1 along the line 2A-2A, showing a second embodiment of the invention,

Fig. 3 is a block flow diagram for a process for applying a protective coating to a substrate and

Fig. 4 is a graph showing coating performance as a function of coating composition.

Fig. 1 shows a component of a gas turbine engine, such as a turbine blade or vane, which in this case is depicted as a turbine blade 20. The turbine blade 20 includes a vane 22 against which the flow of exhaust gas The turbine blade 20 is mounted on a turbine disk (not shown) by means of a dovetail 24 which extends downwardly from the vane 22 and engages a slot of the turbine disk. A platform 26 extends longitudinally outside the area where the vane 22 is connected to the dovetail 24. A number of cooling channels optionally extend through the interior of the vane 22 and terminate in openings 28 in the surface of the vane 22. A flow of cooling air is directed through the cooling channels to reduce the temperature of the vane 22.

The airfoil 22 of the turbine blade 20 is protected by a protective coating 30, two embodiments of which are illustrated in Figures 2A and 2B. In each case, the protective coating 30 is present on a surface 31 of the turbine blade 20 which serves as a substrate 32 for the protective coating 30.

In the embodiment of Fig. 2A, the protective coating 30 includes a platinum aluminide region 34 disposed at the surface 31 of the substrate 32. In the embodiment of Fig. 2B, the protective coating 30 includes a platinum aluminide region 36 at the surface 31 of the substrate 32 and a ceramic thermal barrier layer 38 overlying the platinum aluminide region 36. The protective coating 30 shown in Fig. 2B includes the metallic region 36 (referred to as a bond coat in this context) and the ceramic layer 38 and is sometimes referred to as a thermal barrier coating system. The two platinum aluminide regions 34 and 36 may be of the same or different structures and compositions within the scope of the invention. The platinum aluminide regions 34 and 36 preferably have a thickness of 0.038 mm (0.0015 inches) to 0.102 mm (0.004 inches), most preferably they are 0.0635 mm (0.0025 inches) thick.

3 is a block flow diagram of a preferred process for making the protective coatings of FIGS. 2A and 2B. In a first step, 50, the substrate 32 itself is created. The substrate is a nickel-base superalloy, preferably an advanced third generation, single crystal nickel-base superalloy containing significant amounts of both aluminum and rhenium. The substrate is essentially single crystal, although small amounts of polycrystalline material are tolerated. The aluminum content is from 5 to 16 wt.%, most preferably 6 to 7 wt.%, in such advanced superalloys. At least 5 wt.% aluminum is present to produce a sufficiently large volume fraction of the strengthening γ' phase. The rhenium content is from 1 to 8 wt.%, most preferably 2.5 to 6 wt.%, in such advanced superalloys. A most preferred substrate is a single crystal substrate made from alloy RN5 having a composition in wt.% of 7.5% cobalt, 7% chromium, 6.2% aluminum, 6.5% tantalum, 5% tungsten, 1.5% molybdenum, 3% rhenium, balance nickel. Optionally, some yttrium and/or hafnium may be present. The approach of the invention is also feasible with other advanced alloy substrates such as alloy RN6 having a composition in wt.% of 12.5% cobalt, 4.5% chromium, 6% aluminum, 7.5% tantalum, 5.8% tungsten, 1.1% molybdenum, 5.4% rhenium, 0.15% hafnium, balance nickel, and alloy R142 having a composition in wt.% of 12% cobalt, 6.8% chromium, 6.2% aluminium, 6.4% tantalum, 4.9% tungsten, 1.5% molybdenum, 2.8% rhenium, 1.5% hafnium, balance nickel.

The optimized platinum aluminide coating of the invention exhibits excellent performance on a wide variety of substrate materials, but this improved performance is particularly important for these advanced nickel-based single crystal alloy substrates. These advanced single crystal alloy substrates have higher aluminum content than previous nickel-based superalloys, resulting in a greater amount of gamma-' phase, approximately 60-70 vol.%, than previous nickel-based superalloys. They are used at higher operating temperatures, above 1093.3ºC (2000ºF), than previous nickel-based superalloy substrates, and diffusion effects are correspondingly more important. The platinum aluminide coating of the invention does not exhibit excessive coating growth, surface roughening, production of undesirable phases during use, or reduced stress rupture properties during use at such high temperatures. The combination of such an advanced nickel-based single crystal alloy substrate and the platinum aluminide coating described next is the most preferred embodiment of the invention. The platinum aluminide coating is not limited to use on such advanced single crystal superalloys.

A platinum layer is deposited on the surface of the substrate 32 as shown in step 52. The platinum layer is preferably deposited by electroplating, but other feasible techniques such as sputtering and chemical vapor deposition of organometallic compounds may also be used. The platinum layer desirably has a thickness of 0.0076 mm (0.0003 inches).

Platinum from the platinum layer is diffused into the surface of the substrate by heating the substrate and the deposited platinum layer, step 54. The preferred diffusion treatment is at 982-1093.3ºC (1,800-2,000ºF) for 2 hours. Steps 52 and 54 can be performed simultaneously or sequentially.

A source of aluminum is provided by any feasible technique, step 56. Preferably, a hydrogen and halide gas is contacted with aluminum metal or an aluminum alloy to form the corresponding aluminum halide gas. The aluminum halide gas is contacted with the previously deposited platinum layer overlying the substrate and an aluminum layer is deposited over the platinum layer. The reactions take place at elevated temperature so that aluminum atoms transferred to the surface diffuse into the surface of the platinum-enriched region and the substrate, step 58. Steps 56 and 58 are therefore typically carried out simultaneously.

The temperature of the treatment, the composition of the source, the exposure time and the amount of aluminum source gas determine the amount of aluminum transferred to and diffused into the substrate. The activity of the aluminum is determined using a pure nickel foil of 0.025 mm thickness, which is placed in the aluminization reactor at the same locations where substrates are to be placed. This avoids complications associated with the measurement of aluminum in multicomponent systems. The Foil is treated in the reactor to saturate with aluminum. The aluminum content of the foil is measured by acid dissolution and analysis by a suitable method such as inductively coupled plasma emission spectroscopy. From these measurements, the processing in the aluminizing treatment was determined. The preferred treatment produces an activity between 40 and 50 atomic percent in a pure nickel foil. In a preferred approach, the aluminizing and diffusion treatment are carried out at a temperature of 1051.7-1121ºC (1,925-2,050ºF) for 4-16 hours.

After completion of the diffusion treatment, the chemical compositions of the platinum-aluminide region 34, 36 and the portion 36 of the substrate 32 immediately adjacent to the platinum-aluminum region 34 vary as a function of depth below the surface. The aluminum content and platinum content of the platinum-aluminum region 34, 36 is relatively high adjacent to the surface 31 and decreases with increasing depth into the region 34, 36 and the substrate 32. The remainder of the composition, totaling 100 wt.%, is made up of components of the bulk composition of the substrate alloy and is high at a great depth below the surface 31 and decreases to a lesser value immediately adjacent to the surface 31.

Because of this variation in composition with depth, the compositions of the surface regions are measured by an integration technique. The coated substrate is cut perpendicular to the surface. The weight percent of aluminum, platinum, and other elements of interest are determined as a function of distance from the surface by any technique that yields local compositions, such as an electron microprobe with a dispersive wavelength spectrometer or a dispersive energy spectrometer (in conjunction with appropriate calibration standards). The measurements are made with an electron grid that produces a window at least 5 µm x 5 µm in size. Such composition measurement techniques are known in the art. Composition measurements are made at locations starting within 2-3 µm of the outer exposed surface and then in increments of 5 µm or less from the previous measurement. The wt% content of the element of interest is plotted as a function of distance from the outer exposed surface to a maximum distance which serves as the upper limit of integration. The upper limit of integration is chosen as the distance at which the wt% aluminum has decreased from the higher values closer to the surface to 18%, because below 18% aluminum the β-NiAl is not stable. The area under the curve is determined using any suitable technique, such as trapezoidal approximation, and divided by the value of the upper limit of integration.

Extensive testing was undertaken, as described in more detail below, to determine the characteristics, properties and treatment of the optimal platinum-aluminum region 34, 36. The result is that the region 34, 36 has an integrated composition of 18 to 24 wt.% aluminum and 18 to 45 wt.% platinum. More preferred is the integrated composition of 21 to 23 wt.% aluminum and 30 to 45 wt.% platinum. The remainder of the composition is diffused components of the substrate, mainly nickel, cobalt and chromium, so that the total composition of aluminum, platinum and the diffused components is 100%.

This region 34, 36 is a single phase, relatively ductile composition of aluminum, platinum, nickel and the diffused components of the substrate. In the preferred approach, the region 34, 36 has a thickness of 0.0635 mm (0.0025 inches).

Following the process of Fig. 3 described up to this point, one or both of two additional processing steps may optionally be performed. The substrate 32 and the interdiffused region 34, 36 may be annealed to remove stress from the interdiffused region 34, 36, step 60. This annealing, although widely used for certain protective coatings, has not been found to be necessary in the present approach. If used, a preferred annealing treatment is at a temperature of 982-1043.3°C (1800-2000°F) for a time of 1/4 to 2 hours.

Optionally, a ceramic layer may be deposited on the surface 31 of the substrate 30, step 62, if the final structure is to be that of a thermal barrier coating system of the type depicted in Figure 2B. The ceramic layer for a thermal barrier coating 38 is preferably yttria stabilized zirconia (YSZ) having a composition of zirconia and about 6-8 wt.% yttria and a thickness of 0.127-0.381 mm (0.005-0.015 inches). The YSZ is deposited by any practical technique, most preferably by electron beam physical vapor deposition.

Coatings of a variety of platinum-aluminum compositions were prepared according to the preferred approach described above using RN5 substrates. The coated samples were tested in burner racks in a high velocity environment with 0.5 ppm salt at 1129ºC (2150ºF). The lives of the coated samples were determined in hours of exposure per 0.254 mm (0.001 inch) of coating. Figure 4 shows the results of these tests. There is a distinct region of significantly improved performance for platinum-aluminum regions with an integrated aluminum content of 18 to 24 wt.% and an integrated platinum content of 18 to 45 wt.%, balance components of the bulk composition of the substrate. Particularly desirable results are obtained for an optimal composition range where the integrated aluminum content of the surface region is from 21 to 23 wt.% and the integrated platinum content of the surface region is from 30 to 45 wt.%. Outside these limits, the protection provided by the surface region decreases.

This invention has been described in connection with specific embodiments and examples. However, those skilled in the art will recognize various modifications and variations of which the present invention is capable without departing from the scope as represented by the appended claims.

Claims (14)

1. An article having a platinum aluminide surface area comprising: a substrate having a nickel-based bulk composition and a substrate surface and a platinum aluminide surface region at the substrate surface formed by depositing a platinum layer on the substrate surface and diffusing the platinum layer into the substrate surface and then providing an aluminum source and diffusing aluminum into the substrate surface, the substrate region having an integrated aluminum content of 18 to 24 wt.% and an integrated platinum content of 18 to 45 wt.%, the remainder being components of the substrate bulk composition, totaling 100 wt.%, and wherein the platinum and aluminum contents are relatively high adjacent the substrate surface and decrease with increasing distance from the substrate surface into the substrate. 2. The article of claim 1, wherein the integrated aluminum content of the surface region is from 21 to 23 wt.% and the integrated platinum content of the surface region is from 30 to 45 wt.%. 3. The article of claim 1 or 2, wherein the article further comprises a ceramic layer overlying the surface region, the ceramic layer having a thickness of 0.127 mm (0.005 inches) to 0.381 mm (0.015 inches). 4. An article according to any preceding claim, wherein the article is a turbine blade or a turbine vane. 5. An article according to any preceding claim, wherein the nickel-based alloy substrate is substantially single crystalline and the substrate bulk composition includes from 6 to 41 wt.% aluminum and from 1 to 8 wt.% rhenium. 6. The article of any one of claims 1 to 4, wherein the nickel-base alloy substrate is substantially single crystalline and the substrate mass composition is selected from the group consisting of (a) 7.5% cobalt, 7% chromium, 6.2% aluminum, 6.5% tantalum, 5% tungsten, 1.5% molybdenum, 3% rhenium, balance nickel; (b) 12.5% cobalt, 4.5% chromium, 6% aluminum, 7.5% tantalum, 5.8% tungsten, 1.1% molybdenum, 5.4% rhenium, 0.15% hafnium, balance nickel and (c) 12% Cobalt, 6.8% chromium, 6.2% aluminum, 6.4% tantalum, 4.9% tungsten, 1.5% molybdenum, 2.8% rhenium, 1.5% hafnium, balance nickel. 7. A method of making an article having a platinum aluminide surface area, comprising the steps of: Shelling a substrate having a mass composition of a nickel-based alloy and a substrate surface, then Depositing a platinum layer on the substrate surface, then Diffusing platinum from the platinum layer into the substrate surface, then Creating an aluminium source and then Diffusing aluminum from the aluminum source into the substrate surface for a time sufficient to create a surface area at the substrate surface, the substrate area having an integrated aluminum content of 18 to 24 wt.% and an integrated platinum content of 18 to 45 wt.%, the remainder being components of the substrate bulk composition. 8. The method of claim 7, wherein the step of diffusing aluminum includes the step of heating the substrate surface and the aluminum source to a temperature of 1051.7-1121°C (1925-2050°F) for a period of 4 to 16 hours. 9. A method according to claim 7 or 8, wherein the step of providing a source of aluminum includes the step of providing a source of aluminum in contact with the substrate surface, the source having an activity of 40 to 50 atomic percent as measured in a pure nickel foil. 10. The method of claim 7, 8 or 9, wherein the step of diffusing platinum includes the step of heating the substrate and the platinum layer to a temperature of 982-1093.3°C (1800-2000°F) for a period of 2 hours. 11. A method according to any one of claims 7 to 10, including, after the step of diffusing aluminium, an additional step of depositing a ceramic layer overlying the substrate surface. 12. The method of any one of claims 7 to 11, wherein the step of diffusing aluminum includes the step of diffusing aluminum from the aluminum source into the substrate surface for a sufficient time such that the surface region has an integrated aluminum content of 21 to 23 wt.% and an integrated platinum content of 30 to 45 wt.%, balance components of the substrate bulk composition. 13. The method of any one of claims 7 to 12, wherein the step of providing a substrate includes the step of providing a substrate of a nickel-based alloy that is substantially single crystalline and has a composition containing from 6 to 41 wt.% aluminum and from 1 to 8 wt.% rhenium. 14. The method of any one of claims 7 to 12, wherein the step of providing a substrate includes the step of providing a nickel-base alloy substrate that is substantially single crystalline and has a composition selected from the group consisting of (a) 7.5% cobalt, 7% chromium, 6.2% aluminum, 6.5% tantalum, 5% tungsten, 1.5% molybdenum, 3% rhenium, balance nickel; (b) 12.5% cobalt, 4.5% chromium, 6% aluminium, 7.5% tantalum, 5.8% tungsten, 1.1% molybdenum, 5.4% rhenium, 0.15% hafnium, balance nickel and (c) 12% cobalt, 6.8% chromium, 6.2% aluminium, 6.4% tantalum, 4.9% tungsten, 1.5% molybdenum, 2.8% rhenium, 1.5% hafnium, balance nickel.